1. Field of the Invention
The present invention relates to an inverter device for driving alternating current loads such as a.c. motors. More particularly, it relates to an improvement in an overload protecting circuit for an inverter device.
2. Discussion of the Background
FIG. 1 shows a conventional inverter device in which a reference numeral 1 designates an a.c. power source connected to a converter 2 constituted by rectifying elements 3; a numeral 4 designates a capacitor connected on one hand to the converter 2 and connected on the other hand to an inverter 6 through a current detector 5; a numeral 7 designates an a.c. motor connected to the inverter 6; a numeral 8 designates a control circuit for outputting a first gate signal 9 to an interruption circuit 10; a numeral 11 designates a second gate signal as an output of the interruption circuit 10 fed to the inverter 6; a numeral 12 designates a current-detection signal as an output of the current detector 5 to be input into an amplifier 13; a numeral 14 designates a current as an output of the amplifier 13 to be input into an overload discrimination circuit 15 as an overload protecting means; and a numeral 16 designates an overload-interruption signal as an output of the overload discrimination circuit 15 to be input into the interruption circuit 10.
The operation of the conventional overload protecting circuit will be described.
The converter 2 rectifies an alternating current from the a.c. power source 1 by the rectifying elements 3 to form a direct current. The direct current resulted by the converter 2 is smoothed by the capacitor 4 and the smoothed direct current is fed to the inverter 6, in which electric valves connected the arms of the inverter 6 are sequentially turned on and off by a signal which has been subjected to pulse width modulation by the second gate signal 11. Thus, the inverter supplys an a.c. power having a variable frequency and a variable voltage to the a.c. motor as an alternating current load to drive it at a variable speed.
The control circuit 8 outputs the first gate signal 9 subjected to pulse width modulation. When the interruption circuit 10 is driven under normal condition, namely, when no overload-interruption signal 16 is output, the first gate signal 9 is output from the circuit 10 as the second gate signal 11 as it is (the magnitude of the input signal and the output signal is same.). However, when the overload-interruption signal 16 is output, the second gate signal 11 produces a signal for turning off the electric valves of the inverter 6 irrespective of the first gate signal to break supply of power to a load. In this case, the current detector 5 detects current fed to the inverter 6 and input a detection signal 12 having the magnitude corresponding to the detected current to the amplifier 13. The amplifier 13 amplifys the current detection signal 12 and inputs the amplified signal to the overload discrimination circuit 15 as a current signal 14. The overload discrimination circuit 15 possesses inverse time-current characteristic as shown in FIG. 2. Namely, the overcurrent interruption signal 16 is not output when a current flowing in the inverter is less than a first rated current 17 which is the rated value of the inverter. However, when an overload condition such that a current exceeding the first rated current 17 flows for a predetermined time period is maintained, the overload-interruption signal 16 is output to the interruption circuit 10 to protect elements and devices such as the converter 2, the capacitor 4, the inverter 6, the a.c. motor 7, after lapse of time corresponding to intensity of the current. When the overload-interruption signal 16 is output, the interruption circuit 10 outputs to the inverter 6 a signal for turning off the electric valves of the same whereby the inverter 6 is stopped to supply electric power to the load.
In the overload protecting circuit of the conventional inverter device having the construction described above, when the converter 2 and the smoothing capacitor 4 are designed in consideration of the first rated current 17 on the basis of use of three phase alternating power as the power source 1, if a single phase alternating power is used as the power source 1, an excessively increased current flows in the converter 2 and the smoothing capacitor 4 because of change in phase from three phases to a single phase and the increased current exceeds a designed value. Accordingly, it is necessary for the rated current to be smaller than the first rated current for the three phase alternating power and the converter 2 and the smoothing capacitor 4 have to be used at a current region lower than a second rated current for a single phase alternating power so that current flowing in these devices is maintained within a designed value. However, it is difficult to protect the converter 2 and the smoothing capacitor 4 from breakage caused by overload at the time of feeding the single phase power since the overload protecting circuit 15 has inverse time protective characteristic which is determined on the basis of the first rated current for the three phase alternating power.